We propose to continue our studies of the structure and function of animal mitochondrial DNA (mtDNA). The structure of these DNAs is investigated in mapping experiments, using restriction enzymes and electron microscopy. We plan to complete mapping the sites for ribosomal RNA and transfer RNA on the mtDNAs of the frogs Xenopus laevis and X. mulleri. We propose to isolate poly(A) containing RNAs from Xenopus mitochondria and characterize these presumptive messenger RNAs. We then plan to map the gene regions for major components of these RNAs on the mtDNA. We plan to study the synthesis of the enzyme cytochrome c oxidase in frog mitochondria and to compare the enzyme in the two Xenopus species. We propose to study the mtDNA in mammalian cultured cell lines which carry cytoplasmically inherited markers like chloramphenicol resistance. We plan to test the possible recombination in mtDNA molecules in hybrid cells between different cell lines with different cytoplasmic markers. BIBLIOGRAPHIC REFERENCES: Dawid, I. B., Klukas, C. K., Ohi, S., Ramirez, J. L., and Upholt, W. B. (1976). Structure and Evolution of animal mitochondrial DNA. In, The Genetic Function of mitochondrial DNA, C. Saccone and A. M. Kroon, eds. pp. 3-13. Amsterdam: North-Holland. Upholt, W. B., and Dawid, I. B. (1976). Functional Organization and Evolution of Animal Mitochondrial DNA. Th. Bucher et al. eds. pp. 587-592. Elsevier/North-Holland Biomedical Press. Amsterdam, The Netherlands.